1. Field of the Invention
The present invention relates to rep-rate variable, ultrafast optical sources that have energies suitable for applications in high-energy, ultrafast laser systems. These sources may replace mode-locked lasers for generating short pulses for higher energy applications. The sources are based on inexpensive longer pulse sources and utilize pulse compression in optical fibers to obtain shorter pulses at high pulse energies.
2. Description of the Related Art
The present invention relates to a source for high-energy, ultrafast lasers. Most of the related art is focused on sources for telecommunications. Presently, there is a push to develop technology that allows an increase in the data to be transmitted by a fiber for telecommunications. There are two means for increasing the data rate. One is by increasing the number of channels, where the channels are at different wavelengths (WDM). The other is by increasing the data rate per channel by increasing the frequency of the data (TDM). Presently installed systems typically run at 10 Gigabits-per-sec (Gbit/s) and below, however, there is significant progress in developing systems that operate at 40 Gbit/s and 160 Gbit/s. The present state of the art is an experimental system that operates at 1.28 Terabit-per-sec (Tbit/s) for one channel (Nakazawa et al, “Ultrahigh-speed OTDM Transmission beyond 1 Tera Bit-Per-Second Using a Femtosecond Pulse Train” WECE Trans. Electron. E38-C, pp. 117-125, (2002)).
There are many technical challenges in increasing the frequency of telecommunication systems. The one that is relevant here is the optical source of the high frequency pulses. The present optical source is a cw laser diode having its output modulated with a lithium niobate amplitude modulator. The laser diode can be directly modulated, however, direct modulation of the diode typically imposes a spectral chirp on the laser diode output that degrades the signal after propagation some distance down a fiber. It is not certain that lithium niobate modulators and the related electronics will be able to reach the frequencies and picosecond and subpicosecond pulse widths needed for the systems of the future. Therefore research on alternative sources is presently very active. The alternative sources can be categorized into three areas. The first two are laser diode based devices where the thrust of the research is to improve the fidelity of the pulses generated. One category of such devices are mode-locked laser diodes where the frequency is determined by the round trip time of the laser cavity. The other category of laser diode based devices is gain switched laser diodes where the frequency is determined by the electronics. In order to get short pulses from gain switched diodes, the pulses need to be compressed after the diodes. This is normally accomplished by soliton compression in fibers.
The third category of sources are mode-locked fiber lasers. Mode-locked fiber lasers generally give high quality pulses but operate at lower frequencies than 40-160 GHz. The reason for the lower repetition rate is there is normally only one pulse in the cavity of a mode-locked laser and the cavity of the fiber laser needs to be long for sufficient gain from the fiber. The thrust of mode-locked fiber laser research is to increase the frequency of these devices by methods such as higher harmonic mode locking.
The configuration that is related to the present invention is the gain switched diode followed by a fiber for soliton pulse compression. An early example is in (Ahmed et al, “Generation of 185fs pedestal-free pulses using a 1.55 μm distributed feedback semiconductor laser” Electronic Letters 31, pp 195-196, (1995)). The potential to generate variable and low rep rates from gain switched diodes with external pulse compression would be advantageous for using these devices in high-energy systems. The soliton pulse compression technique normally used is adiabatic soliton compression in dispersion decreasing fiber. A dispersion decreasing fiber is a fiber that has its core slowly decreased. In order to remain a soliton with decreasing dispersion the pulse width must slowly decrease. Pulse compression in dispersion decreasing fiber usually gives good pulse quality and pulse compression factors up to 16. A disadvantage with the present telecom gain-switched diode designs is due to the low pulse energies required and generated. Under these conditions, the nonlinearity for fiber pulse compression is small and so the fibers are usually quite long and expensive, particularly if the fiber is dispersion decreasing fiber. Often a fiber that corrects the chirp of the laser diodes is also required before the dispersion decreasing fiber. This is often near a kilometer long. In addition, a nonlinear optical device is also often required to remove a long pulse pedestal after the fiber pulse compressor. Such a device is described in (K. Tamura et al, “50 GHz repetition-rate, 280-fs pulse generation at 100 mw average power from a mode-locked laser diode externally compressed in a pedestal-free pulse compressor” Optics Letters, 27 pp. 1268-70 (2002)) This three element compressor in addition to the diode makes these systems expensive.
The desired properties for the optical sources of this invention are the ability to produce picosecond and subpicosecond pulses with variable repetition-rates and with energies suitable for further amplification to create energetic, ultrafast pulses. Another desired feature is low cost. These sources will be used in ultrafast sources that have many applications. A few of the applications now being pursued are femtosecond micromachining, refractive index alteration in transparent materials for optical memory and photonic devices, three-dimensional integrated optics and photonic crystals, eye surgery, dentistry, dermatology and microsurgery. For these applications, the pulse characteristics are quite different than for telecom systems. Instead of picojoule pulse energies and >1 GHz repetition rates, pulse energies in the microjoule to millijoule range are desired with repetition rates from 1 kHz to 1 MHz. Chirped pulse amplification is used to accommodate the high energies in the fiber amplifier. In chirped pulse amplification the pulse is first spectrally chirped and thus temporally lengthened to keep the peak power lower in the fiber during amplification. After amplification, the pulse is recompressed. Chirped pulse amplification in fibers is described (Galvanauskas, “Method and Apparatus for generation high energy ultrashort pulses” U.S. Pat. No. 5,400,350). The source in this patent is a laser diode that is electronically chirped to give a 1 ns pulse that was amplified and then compressed to ˜2 ps. It is highly desirable to be able to obtain even shorter pulses. The source of pulses for chirped pulse amplification has been predominately femtosecond mode-locked fiber or solid-state lasers that operate at 50-100 MHz. These sources are typically down-counted; e.g., for 1 kHz operation, one pulse out of 50,000-100,000 is amplified. A source that could be operated at variable and lower frequencies would be more suitable.
Femtosecond mode-locked fiber lasers do not normally have sufficient pulse energies and the pulses are often longer than desired for these nontelecom sources. Soliton compression (narrowing) during amplification in a fiber amplifier and higher-order soliton compression have already been utilized with these sources. Soliton narrowing during amplification is equivalent to decreasing the dispersion in the fiber. As the pulse peak power is increased the pulse width needs to decrease to maintain the soliton. Such pulse compression for higher-energy pulses is described in (Fermann, Galvanauskas and Harter, “Apparatus and Method for the Generation of High-Power Femtosecond Pulses from a Fiber Amplifier” U.S. Pat. No. 5,880,877). The fiber amplifier can be less than a meter and soliton compression is built into this amplifier. A pulse is seeded into a fiber amplifier and it is amplified to energies of higher order solitons. As the higher order soliton propagates in a fiber its pulse width is periodic. During this periodic evolution, the pulse initially contracts by a factor dependent on the order of the soliton. It is this phenomenon that is used for compression. A compression factor of 100 can be obtained by this method but typically a smaller factor is used since pulse energy and length becomes too sensitive. A means to get to even higher pulse energies with soliton compression is described in U.S. Pat. No. 5,880,877. Higher energies are possible by utilizing a multimode fiber to propagate a single transverse mode. The intensity in the fiber is decreased since the multimode fiber has a larger mode area for the fundamental mode compared to a single mode fiber. Thus, higher pulse energies are necessary before soliton effects again become important.
An alternative to soliton compression in an optical fiber with negative group velocity dispersion (GVD) is pulse compression with a fiber having positive GVD. Just as with soliton compression, there is a balance of dispersion with self-phase modulation in the fiber. There is simultaneously spectral broadening of the pulse by self-phase modulation with temporally stretching of the pulse to give a linear spectral chirp by dispersion. After the fiber, the chirped pulse is recompressed. The first experiments in compression of ultrashort pulses with optical fibers were accomplished in this manner. In the first experiment by (Nakatsuka et al, “Nonlinear Picosecond-Pulse Propagation through Optical Fibers with Positive Group Velocity Dispersion”, Physical Review Letters 47, pp. 910-913 (1981)), 5.5 picosecond pulses from a mode-locked dye laser were compressed to 2 ps giving a compression factor of 2.75. In the following six years significant progress was made in pulse compression utilizing this method until (Fork et al, “Compression of optical pulses to six femtoseconds by using cubic phase compensation” Optics Letters 12 pp. 483-5 (1987)) pulses were compressed to the long-standing record of 6 femtoseconds. The maximum pulse compression demonstrated is around 110× in one stage of compression. (Dianov, “Generation of high-contrast subpicopulses by single-stage 110-fold compression of YAG:Nd3+ laser pulses”, Soviet Journal of Quantum Electronics, 17, pp. 415-416, (1987). Compression factors as high as 450 have been reported with a two stage fiber-grating compressor in (Zysst et al, “200-femtosecond pulses at 1.06 μm generated with a double-stage pulse compressor” Optics Letters 11 pp. 156-8 (1986)). This compression method has been commercialized for pulse compression of cw mode-locked Nd:YAG lasers operating at 1.06 μm. The pulse widths from these lasers are between 30-100 ps and the pulses are compressed normally by a factor of 100 to the subpicosecond range. The details of these systems can be found in (Kafka et al, “Pulse compression” U.S. Pat. No. 4,750,809, Kafka et al, “Peak power fluctuations in optical pulse compression”, U.S. Pat. No. 4,896,326 and Kafka et al “Optical fiber for pulse compression” U.S. Pat. No. 4,913,520). This compression method has not been applied to gain switched diodes for telecom systems since fibers at the telecom wavelengths (˜1.5 μm) are not normally positively dispersive, and soliton compression is less sensitive to amplitude fluctuations and does not require additional gratings for compression. However, the most important factor is the required peak power. For this compression method orders of magnitude higher pulse energies are required compared to soliton compression.
More recently, pulse compression utilizing positively dispersive amplifying fiber has generated nanojoule-range pulse energies for non-telecommunication applications. One method is described in U.S. Pat. application Ser. No. 09/576,772. This application describes primarily the use of parabolic pulse amplification. It does describe some pulse compression (2-10×) for seed pulses with a pulse width of 0.2-1 ps. It does not describe the pulse compression of pulses longer than 1 picosecond such as are generated from laser diodes or microchip lasers. (M. E. Fermann, A. Galvanauskas and D. Harter, “Single-mode amplifiers and compressors based on multimode optical fibers”, U.S. Pat. No. 5,818,630) also teaches using positive GVD MM amplifier or undoped fibers for pulse compression. It does not teach parabolic pulse amplification or pulse compression for the pulses from gain switched laser diodes or microchip lasers that are the initial sources for the rather long pulses. For higher energies pulse compressors using positive GVD fibers that are multimode have been used. Pulse compressors utilizing multimode, positive GVD graded-index fibers has been used for initial pulse energies as high as 2 microjoules in (Damm et al, “Compression of picosecond pulses from a solid-state laser using self-phase modulation in graded-index fibers”, Optics Letters 10, pp. 176-8, (1985)). However, in this case, the output was multi-transverse mode. Pulse compressors utilizing multimode, positive GVD fibers with single-mode output are described in (Fermann and Harter, “Single-mode amplifiers and compressors based on Multi-mode fibers”, U.S. Pat. No. 5,818,630). The highest pulse energy utilized in compression with single mode operation of multimode fibers was in the nanojoule regime.
The sources of the initial pulses in this invention are gain switched laser diodes and microchip lasers. Gain switched laser diodes are described in the previously mentioned paper by Ahmed et al. A microchip laser is a small diode pumped solid-state laser. The microchip is either actively Q-switched or passively Q-switched. A commonly used passively Q-switch design is given in (Zayhowski et al., “Diode-pumped passively Q-switched picosecond microchip lasers”, Optics Letters 19, pp. 1427-29 (1994)). The microchip laser that was used in this invention is described in (Zayhowski et al., “Coupled-cavity electro-optically Q-switched NdYVO4 microchip lasers”, Optics Letters 20, pp. 716-8 (1995)). Pulse widths as low as 115 ps have been demonstrated for this actively Q-switched laser. In (Braun et al., “56-ps passively Q-switched diode-pumped microchip laser”, Optics Letters, 22 pp 381-2, (1997)) 56 ps pulses from a passively Q-switched laser were obtained.